Perishable goods are shipped in refrigerated containers in order to maintain freshness and quality. Refrigerated containers can keep pallets of perishable goods cold by establishing a circulation of air from the container, for example, through a refrigeration unit and back into the container. The refrigeration unit can include or have a standard vapor compression cycle, with an evaporator cooling the air and a condenser rejecting heat to the outside of the container.
Perishable goods, like fruits and vegetables, respire during storage or transport in the refrigerated container. The fruits and vegetables convert oxygen to carbon dioxide as they respire. They also give off or emit other gases, such as ethylene or volatile compounds, such as aldehydes or alcohols or other hydrocarbons, which have characteristic odors and/or can impact ripening. These gases, such as carbon dioxide and ethylene, can build up or increase in volumes and/or quantities in shipping containers. In addition, microbes such as molds or fungus can grow in a container filled with perishable goods. The microbes reproduce by producing relatively high volumes of spores that are transported through the air to new surfaces. Relatively high concentrations of spores in the air have been shown to lead to relatively high concentrations of spores on the surfaces of perishable items.
It is possible to vent the gases and spores by allowing some air in the container to exhaust from the container while pulling in air from outside the container to replace it. Ventilation procedures are well established in the storage and shipping business to lower levels of carbon dioxide and ethylene in the containers.
Ventilation can result in additional energy usage to condition the outside air that is introduced into the container. In addition, the outside air may not be completely clean. It contains humidity, fumes from outside sources of contaminants, and the exhaust gases of nearby containers. Some of these contaminants are not desirable in the refrigerated container. Relatively high ventilation rates can increase the operation of the evaporator, which, in turn, can dry out the air in the container and lead to moisture loss from the perishables.
There is a continuing need for improved air cleaning in storage containers, such as with a reduction or elimination of air venting to save on cooling energy and cost.